The invention relates in particular to the metering zone of such a plasticating screw. In plasticating screws, such a metering zone follows directly on the plasticating zone. This section of the screw, also known as the compression zone, thoroughly mixes the heat-softened granular material and conveys the plasticated material onwards under pressure towards an extruder nozzle. Intermixing in this zone constitutes an important task of an extruder screw, since the plasticated material is still inhomogeneous, that is, as yet unplasticated constituents could be present, which still have to be heat-softened. The inhomogeneity causes different shear forces at the walls of the screw and in the middle of the screw channel, whereby intimate mixing of the material is prevented and additionally pressure fluctuations in the material can arise. To remedy these problems, screws having a tapering screw channel and screws having guide webs in the main spiral have been proposed, for example, as well as plasticating screws having one or more of these above-mentioned features combined.
For example, such plasticating screws inter alia are already described in WO 00/34027, DE 196 34 162 C2 and U.S. Pat. No. 5,599,098. WO 00/34027 describes the principle of increasing the channel depth in conjunction with guide members. In German patent 196 34 162 C2, an auxiliary screw is provided before the end of the plasticating screw in order to keep variations in edge thickness in strip-form material to a minimum. U.S. Pat. No. 5,599,098 describes an extruder screw having a melt section for plastics material, the section consisting of screw channels that partially overlap. In this case, the flow of material at the start of the overlapping screw channels is divided into two partial melts, the flow rate and hence the pressure of the material increasing because of the smaller overall cross-section. The newly starting screw channel is continued, whilst the original screw channel ends after ½-1 turn in the direction of flow. The drawback to this solution is that the available cross-section of the overlapping screw channels, especially at high material through-put, is reduced at least by one screw wall cross-section. At the same time, this is associated with a sudden increase in pressure.